docs: improve README examples of stats/base/dists/invgamma namespace

[ShivamAhir]

Resolves # .

Description

Purpose of this pull request is to add example in stats/base/dists/invgamma namespace

This pull request:

• Add examples of the functions present in stats/base/dists/invgamma namespace
• adds example demonstration using mean ,mode, variance, stdev functions in the README.
• Adds the same demonstration example in examples/index.js.

Related Issues

Does this pull request have any related issues? #1631

This pull request:

• resolves #
• fixes #

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

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PR Commit Message

docs: improve README examples of `stats/base/dists/invgamma namespace`

PR-URL: https://github.com/stdlib-js/stdlib/pull/1808
Ref: https://github.com/stdlib-js/stdlib/issues/1631

Co-authored-by: Philipp Burckhardt <[email protected]>
Co-authored-by: shivam Ahir <[email protected]>
Reviewed-by: Philipp Burckhardt <[email protected]>

Please review the above commit message and make any necessary adjustments.